This invention is broadly related to the field of electronic musical instruments, particularly electronic organs or other electronic musical instruments having a keyboard such as electronic pianos, accordions and the like. The term "organ" as used throughout the specification and claims is intended in a generic sense to include these other electronic musical instruments. In addition, reference to the actuation of key switches or coupler switches and the like is intended to cover the actuation of such switches by whatever means may be employed, such as directly by action of the musician's fingers or indirectly through intervening levers, apertures, switch closings, touch responsive switches, etc.
In the design of electronic organ, an attempt is made to faithfully reproduce as nearly as possible the musical sounds and tones which are developed by true pipe organs in response to the playing of the electronic organ by a musician. In order to simulate as many pipe organ sounds as possible, electronic organs utilize intramanual and intermanual couplers employed with at least two manual keyboards and a single pedalboard. A pair of pedalboards and an even larger number of manual keyboards are used in more complex electronic organs. The manual keyboards generally encompass several octaves and the pedalboards usually one or more octaves. In addition, a typical electronic organ includes a relatively large number of playing stops or tabs which are associated with each of the keyboards to permit selection of different organ voices for the tones produced by those keyboards by changing the timbre, tone quality, and the like.
To generate the tones capable of production by such an organ, a separate stable oscillator could be provided for each of the many tones. This, however, is prohibitively expensive; and because of the tonal interrelationships between all of the various tones, tuning of such an organ and maintaining tuning of such an organ becomes nearly impossible from a practical standpoint. A system known as a top octave frequency synthesizer system (TOS) has been developed which overcomes the need for using a large number of expensive stable oscillators and instead utilizes a single stable oscillator to provide the tones for the top octave of the organ. Divider circuitry then is employed to generate all of the other tones, and tuning of such an organ becomes a relatively simple matter since only a single oscillator or a small number of oscillators are used in the organ.
While a single oscillator and top octave synthesizer can be used for an entire organ, problems occur if several different divider circuits are connected to the same top octave synthesizer output. If some form of synchronization is not used between the different divider circuits from the same top octave synthesizer output, then it is possible that some tones will have phase reinforcement of harmonics and others phase cancellation of harmonics. This results in very unnatural quality music production by the organ. To overcome these disadvantages, a number of different top octave synthesizers have been utilized in an organ, so that different notes for different octaves in the different manuals of the keyboard are produced by different top octave synthesizers. When such synthesizers are dedicated to a block of keys or a particular part of the organ, however, it still is necessary to use a relatively large number of synthesizer circuits.
To reduce the complexity of the organ, top octave synthesizers known as programmable top octave synthesizers have been developed to permit any key closure to produce any tone in the organ from a given synthesizer circuit. When a number of these circuits are used in an organ along with an assignment or control circuit for assigning different top octave synthesizers to different keys as the organ is being played, maximum efficiency in the electronics of the organ is realized so far as the tone generating portion is concerned.
Top octave synthesizers typically are square-wave generators; so that while their output tones are acceptable for strings, they are not acceptable in square-wave form for the production of flute sounds. It is necessary to filter the outputs of the top octave synthesizer circuits to change the square-wave outputs to sinusoidal wave outputs for the reproduction of proper flute sounds from the instrument. If top octave synthesizers having a capability of tonal production over more than an octave (and typically over the entire range of frequencies of the organ) are used in a system, the filters connected to the outputs of the top octave synthesizers necessarily have been required to be extremely wide band filters. Such filters cannot attenuate all harmonics of a tone over the entire tonal range of the organ and as a consequence do not produce the desired quality of flute tones from the instrument.
It is desirable to provide an electronic organ or other electronic musical instrument with a limited number of top octave synthesizer circuits, each capable of producing any note in the full range of notes produced by the organ, and to group the tone outputs from the top octave synthesizers into subgroups, such as octaves; so that more effective filtering of the tones for producing flute tones can be accomplished by relatively narrow band filters.